1. Field of the Invention
The present invention relates to a levitation system of a magnetically levitated train which travels fast by utilizing the principle of magnetically levitating repulsion.
2. Description of the Prior Art
In case of a magnetically levitated train using superconducting magnets, such as shown in FIG. 13, the car body 1 having superconducting magnets 2a, 2b for propulsion and levitation is propelled by a moving magnetic field generated by applying three phase electric current having a sine wave waveform to the propulsion coils 4a, 4b, 4c, 4d, 4e, 4f, 4g, which are mounted in the guideway 3 on the ground. The car body 1 is also levitated by repulsive magnetic flux generated in the levitation coils 5a, 5c, 5e, which are installed in the same guideway.
The detailed theory of the levitation can be appreciated by referring to FIG. 14 which shows the center line 7 of the vertical direction of the superconducting magnet 2a lowered by .DELTA.Z from the center line 6 of the levitation coils 5a, 5b, which are connected in a figure eight arrangement by weight of the train. Therefore, in accordance with moving of the superconducting magnet 2a, induced current proportional to the difference between chronical variation of magnetic flux interlinked with the top portion 5a of the levitation coil and chronical variation of magnetic flux interlinked with the bottom portion 5b flows to the levitation coils 5a, 5b. The levitation is performed by utilization of repulsive magnetic flux generated with the induced current.
The levitation system described above, and the force created by the system with the superconducting magnet which is loaded on the car body, are described in, for instance, a document of the linear drive study group of the Institute of Electrical Engineers of Japan, Document No. LD-89-26. This document details a case in which the levitation coils have only a single layer. Another case in which the levitation coils have double layers is disclosed in JP-A-2-17806 (1990). In the latter case, the double layers of coils are entirely overlapped with each other, and the coils are so composed that circuit constant, inductance L, and resistance R of the first layer and the second layer are mutually different. Since n pieces of the levitation coils are installed corresponding to two poles of the superconducting magnet, higher harmonic waves of spatial mn.+-.l order (m=0, 1, 2, . . . , integer) are observed as chronical mn order higher harmonic waves at a position of the superconducting magnet. For instance, in a case of n=6, spatial higher harmonic waves of 1st, 5 th, 7th, 11th, and 13th order, and chronical higher harmonic waves of 0th, 6th, and 12th order are observed.
In accordance with the above described prior art, since the levitation coils either of a single layer or double layers are entirely overlapped with each other, chronical n order, for instance 6th order, higher harmonic waves having large amplitude are generated. This results in higher harmonic wave effects to the superconducting magnet, and can cause quenching of the superconducting magnet in a worst case situation. The present invention was developed in view of the above described problems.